A gas diluting and testing apparatus is used to analyze, among other things, vehicular exhaust. The apparatus uses a mixing tee to dilute the exhaust gases so that the moisture content of the gases is sufficiently reduced in order to minimize errors due to condensation. Existing mixing tees have a dilution inlet for receiving a dilution gas, a gaseous inlet for receiving the exhaust gases, and an outlet for connecting to external equipment, such as a sampling system.
A primary concern in attaching any equipment to a vehicle exhaust pipe is to avoid any excess pressure or vacuum on the vehicle exhaust pipe. Current standards range from +/-1 in H.sub.2 O (0.25 kPa) to +/-5 in H.sub.2 O (1.25 kPa), depending on the testing program.
A typical flow rate at the mixing tee outlet for some testing programs is about 300-350 ft.sup.3 /min (8.5-10 m.sup.3 /min). However, lower fuel economy vehicles and methanol fuel vehicles require a higher dilution flow rate for accurate testing. As such, the testing of these vehicles may require as much as 700-800 ft.sup.3 /min (20-23 m.sup.3 /min) flow rate at the mixing tee outlet. With existing mixing tees, increased flow rates at the mixing tee outlet result in increased vacuum at the vehicle exhaust pipe. At these increased flow rates, or at even higher flow rates, it becomes more difficult to stay within specification, particularly in more demanding testing programs which require the +/-1 in H.sub.2 O (0.25 kPa) specification.
Also, many testing programs require that the dilution flow rate be monitored during testing. Monitoring the dilution flow rate and the mixing tee outlet flow rate allows calculation of the exhaust flow rate. One way to measure dilution flow rate is with a subsonic venturi. The venturi is located at the dilution inlet. However, due to flow dynamics of the subsonic venturi (and of other flow meters as well), there is a pressure loss between the inlet and the outlet of the venturi. This pressure loss adds to the overall pressure loss between the dilution inlet which is normally at ambient pressure, and the vehicle's exhaust pipe. Again, increased vacuum at the vehicle exhaust pipe may become a problem when testing at higher mixing tee outlet flow rates due to additional pressure losses at the venturi.
Some existing systems use a blower for pressure gain after the dilution inlet venturi to reduce the overall pressure drop, and hopefully keep the exhaust pipe pressure within specification. An example of a mixing tee having an actively controlled blower is described in U.S. Pat. No. 4,823,591 issued to Lewis and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference in its entirety.
Although the above described systems have been used in many applications which have been commercially successful, stricter specifications and higher flow rate demands may become difficult to meet with such systems. Further, systems using active blower control in order to meet specifications are costly; and, tuning such systems may be complicated.